The growth differentiation factor (GDF) subfamily of bone morphogenetic protein (BMP) proteins is implicated in initiating joint formation in limbs. Both Gdf5 and Gdf6 are expressed in pre-joint interzones during limb developments, in stripes where changes in cell death and differentiation produce joints between skeletal elements. Evidence from targeted mutations indicate that Gdf proteins function as signaling molecules to do this; however, the mechanisms by which Gdf5/6 expression occurs specifically in the interzone are unknown. I propose to identify the regulatory elements that control Gdf6 expression, and to use these elements to identify upstream factors controlling interzone expression. This will be accomplished by driving expression of a lacZ reporter genes in vivo using large genomic clones derived from the mouse Gdf6 gene locus. Analysis of the critical regions of this construct for correct Gdf6 expression will lead to the identification of specific control sequences and the factors that interact with them. In addition, I will use these elements to drive expression of other genes in developing joints. This will make it possible to test the function of other genes in joint function, including other members of the BMP family such as Gdf5. The proposed studies will provide important insight into the mechanisms through which signaling proteins are regulated during develop and should provide insight into the normal process of cartilage and synovial joint formation.